Research On The Microstructural Characteristics And Properties Of Cladding Layer And Composite Interface Of W6Mo5Cr4V2 High Speed Steel/35CrMo Low Alloy Steel Composite Roll

Composite roll is a kind of composite, which is made of two types of different special metallic materials. These materials enable to meet different requirements of the roll in either working conditions of the working layer and the core or comprehensive performances, such as high wear resistance, high resistance to surface roughening and internal toughness. Thus, the composite roll has a tendency to gradually replace the conventional roll that is made by single metallic material. Currently, the composite roll is often fabricated by a solid-liquid composite casting method that can combine two different metallic materials successfully. The key lies in the interface combination for these two materials and therefore, the study of the cladding layer and in particular, composite interface of the composite rolls becomes the top priority of research work.This thesis employed the solid-liquid composite casting method to fabricate an experimental composite roll, which selected W6Mo5Cr4V2 high speed steel as cladding layer and 35 CrMo low alloy steel as core. Research work mainly focused on W6Mo5Cr4V2 high-speed cladding layer and composite interface. The microstructures in initial and heat treated states were systematically observed and analyzed by optical microscope(OM), scanning electron microscope(SEM), energy dispersive spectroscopy(EDS) and X-ray diffraction(XRD), and the effects of austenitizing temperature and tempering temperature on the microstructures and mechanical properties tested. The variation of alloying elements and hardness from surface to core in the composite roll was examined and a thermal fatigue test for the inspection of crack initiation and propagation was carried out in a self-restraint thermal fatigue test machine. On the basis of these investigations, the mechanism of the variation of microstructure and mechanical properties of W6Mo5Cr4V2 speed steel cladding layer and composite interface was explored. In general, the main results obtained in this thesis are as follows:The initial annealed microstructure of the W6Mo5Cr4V2 cladding layer is composed of pearlite and carbide that distributed at grain boundary to form a continuous, coarse and network. The net carbide is mainly composed of two different types of M6 C and M23C6 carbides. The matrix grain appeared smaller in outer layer and coarser inside the middle layers. The continuity and integrity of the network carbide was poorer in outer layer and then gradually stronger when approaching core areas. The matrix microstructure within the inner layer is between the outer and middle layers. Hardness in outer and inner layers was higher than that of middle layer.The matrix of the high-speed steel layer transformed into martensite after quenching. With the increasing austenitizing temperature, the network carbides obviously disconnected. The number of undissolved carbides particles on the matrix gradually decreased, but their size increased. These carbides all dissolved in the matrix at 1100℃. Hardness tests indicated that with the increasing quenching temperature, its value gradually increased, reached the highest at 1050℃ and then decreased; With increasing tempering temperature, martensitic morphology gradually changed and remained before 700℃. The precipitation of carbides on the matrix gradually increased before 500℃ and then enlarged over 500℃. The hardness in cladding layer gradually decreased and the impact toughness increased with the increasing tempering temperature.The composite interface can be divided into three regions, i.e., near cladding layer, central layer and near 35 CrMo steel layer. Matrix microstructure of the composite interface in the initial state was fine pearlite. The microstructure appeared smaller near cladding layer and coarser relatively near 35 CrMo steel layer. The matrix within the central layer is between the cladding layer and 35 CrMo steel layer.With increasing austenitizing temperature, the matrix in the composite interface changed from acicular to invisible martensite and the number of undissolved carbides particles near cladding layer was gradually decreased. The hardness within the composite interface increased with the increasing austenitizing temperature from 950℃ to 1050℃ and decreased at 1100℃. With increasing tempering temperature, martensite gradually decomposed, the hardness decreased, the number of carbides particles on the matrix grains increased at first and then increased in their size after 500℃. The thermal fatigue crack originated from the grain boundary carbides in composite interface and iron oxides that located at the last solidified interface. The crack late extend mainly along the grain boundary carbides. In addition, casting defects near the last solidified interface can cause secondary cracking.